The present invention relates to an electronic circuit for oscillating a piezo-electric resonator employing a semiconductor variable capacitance element, the capacitance of which is controlled by the electric charge stored in a floating electrode covered by an insulating film and insulated from the outside. The floaing electrode is disposed on the surface of a semiconductor substrate through the insulating film.
A semiconductor variable capacitance element has heretofore been known as disclosed in a literature "Proceedings Vol. 2 in the 11th International Congress of Chronometry, edited by The French Society of Microtechnorogy and Chronometry, p. 9, 1984".
FIG. 2 is a diagram illustrating a conventional CMOS quartz oscillation circuit employing a semiconductor variable capacitance element. A feedback resistor 22 is connected between the gate and the drain of an inverter 21, and a drain resistor 23 and a quartz resonator 24 are also connected in series between the gate and the drain of the inverter 21. To both ends of the quartz resonator 24 are further connected a drain capacitor 25 and a semiconductor variable capacitance element 26 for adjusting the oscillation frequency. A voltage of a power source 28 is supplied to the inverter 21 through a constant voltage circuit or constant current circuit 27. Therefore, the quartz oscillation circuit consumes a reduced amount of electric current, and the frequency is prevented from changing that is caused by the power source voltage. The frequency is adjusted by changing the capacitance of semiconductor variable capacitance element 26 according to the electric change stored in the floating electrode of semiconductor variable capacitance element. Therefore the capacitance of semiconductor variable capacitance element 26 can be easily varied. Furthermore, the oscillation frequency can be correctly adjusted to maintain very high reliability.
In the conventional CMOS quartz oscillation circuit shown in FIG. 2, the frequency is adjusted by changing the capacitance of the semiconductor variable capacitance element 26. Change of capacitance, however, required positive and negative voltages of more than 10 volts. Furthermore, it is very difficult to change the frequency with respect to a frequency that has been previously adjusted as a reference, and, it is very difficult to correctly return the freuency that was varied to the reference frequency.